1. Field of the Invention
This invention relates to mixers in general and more particularly to a low cost planar double balanced mixer for narrow frequency bandwidths.
2. Description of Related Art
A mixer circuit converts a radio frequency (RF) signal to an intermediate frequency (IF) signal which is the difference of the RF and a local oscillator (LO) signal. The IF frequency is obtained by multiplying the RF signal with the local oscillator (LO) signal.
Passive double balanced mixers are used widely in systems where frequency translation is required. Most of today's mixer designs are designed to perform at moderate to very broad frequency ranges. Although applications exist where multi-octave bandwidth is necessary, many modern radio systems only require relatively narrow band performance. The process for achieving broad frequencies in mixers generally requires multi-layer broadside coupled baluns. Typically, these designs are difficult and expensive to design and manufacture requiring the use of jumpers, bridges, vias and bond wires.
Mixers using multi-layer broadside coupled baluns offer good broadband performance. However, the cost is high. Many applications do not require wide bandwidths. In some instances, unnecessarily wide bandwidths can allow the passage of stray signals and harmonics requiring the use of additional filters. It is advantageous for a mixer to have optimal performance at the specific frequency required by the radio designer.
A continuing need exists for a mixer that has improved electrical performance such as increased isolation and reduced conversion loss. A continuing need also exists for a mixer that has a small overall size that can be manufactured at low cost and that can be easily simulated and developed.